Fluorescent tube lamp are being more and more replaced by retrofit tube lamps employing semiconductor light emitting elements (light emitting diodes, LEDs). Such retrofit lamps usually comprise a housing which is at least partially translucent or transparent, a light engine comprising a plurality of LEDs and an electronic driver for driving the LEDs such that they emit light.
The term “light engine” is commonly used for the assembly comprising the LEDs and a mechanical structure holding the LEDs and including conductive traces and/or wires for supplying the LEDs with electric power from the driver.
Many LED retrofit tube lamps include a light engine having a printed circuit board (PCB) onto which the LEDs are mounted, for example soldered. Such PCBs often are etched from copper sheets laminated onto a non-conductive substrate, resulting in a high consumption of copper and in high material and process costs.
An alternative to printed circuit boards are flexible circuit board as disclosed in international patent applications WO 2012/009838 A1, WO 2012/009839 A1, WO 2012/009840 A1, WO 2012/009841 A1, and WO 2012/009842 A1, the contents of which are incorporated herein in their entirety. These flexible circuit boards are called “wiring boards” by the applicant of these patent applications. This term will be used in the following. Wiring boards are produced by laminating strips of conductive material (e.g. aluminum, made for example by rolling a round wire into a flat strip) between flexible isolating sheets (e.g. polyimide). The circuit design is then obtained by punching holes into the laminate at predetermined positions, thus separating the conductive strips into a plurality of strip elements. Electronic components can then be connected to the strip elements. Preferably, the upper isolating sheet comprises openings where the underlying metal strip is accessible.
The wiring board is flexible and can be wound on reels. It can be cut to a desired length before or after the electronic components have been placed on and fixed to the substrate.
A known light engine for retrofit tube lamps is shown in FIG. 1 and employs a substrate 1 having two conductive traces 2, 4 running continuously along the longitudinal direction L. “Longitudinal direction” here and in the following means the direction in which the substrate (etched PCB, wiring board, or other) has its largest extension, also known as length. The length of a substrate for a retrofit tube lamp is usually less than approximately 550 mm for a tube lamp having an overall length of 590 mm (also called a 2-feet lamp), less than approximately 1160 mm for a tube lamp having an overall length of 1200 mm (also called a 4-feet lamp), and less than approximately 1460 mm for a tube lamp having an overall length of 1500 mm (also called a 5-feet lamp), leaving enough room for an electronic driver.
The other two perpendicular extensions are the width and the thickness of the substrate. The width of the substrate for the example shown in FIG. 1 is usually approximately 7 mm. The thickness of the substrate depends on the production method and can be between approximately 0.05 mm and 0.5 mm (for example between approximately 0.1 mm and 0.3 mm for some wiring boards, between approximately 0.07 mm and 0.3 mm for some etched PCBs).
The two continuous conductive traces 2, 4 may by electrically connected to the outputs of the electronic driver, thus supplying the operating voltage along the length of the light engine. The known light engine further has a third conductive trace 3 running along the longitudinal direction and arranged between the aforementioned conductive traces 2, 4. The third conductive trace 3 is divided into a plurality of trace sections 3′ (e.g. by etching or punching) such that the trace sections 3′ are arranged one after the other in the longitudinal direction and are electrically isolated from each other (as long as no electronic components are attached) by gaps 5 (also called “separations” in the following).
LEDs 6 are arranged on the substrate such that each LED 6 bridges a gap 5 between two adjacent trace sections 3′. Each LED 6 is electrically connected (e.g. soldered) with its anode to one trace section 3′ and with its cathode to the next (in the longitudinal direction) adjacent trace section 3′. Thus, the LEDs 6 are electrically connected in series. A zero-ohm resistor 7 (i.e., an electrically conductive element having negligible electrical resistance) connects the first continuous conductive trace 2 used for power supply to the first trace section 3′ of the LED series circuit. Another zero-ohm resistor 8 connects the second continuous conductive trace 4 used for power supply to the last trace section of the LED series circuit.
This pattern can be repeated on the substrate 1 and the substrate 1 can be cut along the potential cut line 9 to obtain a light engine having a specific length. Due to the series connection of the LEDs 6, the distance between to potential cut lines 9 is rather large which makes the design inflexible and difficult to use for different lamp lengths.
Furthermore, arranging the LEDs along the longitudinal direction causes enhanced stress on the soldering joints when the light engine is being moved (which happens easily during assembly of a lamp using the light engine).